When the ailerons are moved, they have a secondary effect besides that of moving the aircraft in the rolling plane. This is due to the resistance that they offer to the airflow, i.e. to their drag. The aileron which is depressed obtains more lift due to its increased angle of attack, but this involves more drag. This extra drag tends to turn the aircraft in the yawing plane in the opposite direction to that in which the bank is applied, i.e. if the stick is moved to the left the aircraft yaws to the right. This effect is known as “aileron drag”. It can be minimised by various design features but is still fairly pronounced on most gliders.
Another effect follows from the use of the ailerons. When the aircraft is banked it tends to slip in towards the lower wing. This produces a sideways pressure of air on the fuselage and fin; since there is more effective surface behind the centre of gravity than in front, the result is a yawing movement towards the lower wing. This “weathercock ” effect is in the opposite direction to that produced by aileron drag.
The rudder also has a secondary effect. When an aircraft is yawed by using rudder, the outer wing moves faster than the inner wing. The greater speed of the airflow past the outer wing gives it more lift, so that it rises, causing a movement in the rolling plane, i.e. banking. When the rudder is applied by itself, the outward skid also contributes to this effect, owing to the lateral stability of the aircraft.
The elevator has no secondary effects; the elevator pitches the glider. Of course, if the pitch is changed and the wing’s angle of attack reaches the stalling angle, the glider will stall. More about stalling later.
A glider is designed to be moderately stable in flight; this means that it should tend to keep the same attitude as that in which it is set, and to return to it if it is displaced by small air disturbances. This saves the pilot much effo rt, and tends to make the aircraft “fly itself”.
Stability in the rolling plane – This is normally achieved by setting the wings to the fuselage at a slight upward “dihedral” angle:
Should the aircraft, by reason of some air disturbance, become banked while flying straight, it will begin to slip towards the lower wing, like this:
Stability in the yawing plane – This is provided by the fin and the sides of the fuselage. As the greater part of these surfaces is behind the centre of gravity, the aircraft possesses directional stability, and if it is displaced in the yawing plane will tend to “weathercock” back again.